Variable gear ratio steering systems are widely known in the art, and are known to reduce the effort required to steer the vehicle in a low speed range (or for a large steering angle) and to improve the stability of the vehicle in a high speed range (or for a small steering angle). Such a system typically provides a low gear ratio (in the sense that the steering angle of the road wheels is relatively large for a given steering angle of the steering wheel) for a large steering angle, and a high gear ratio (in the sense that the steering angle of the road wheels is relatively small for a given steering angle of the steering wheel) for a small steering angle. Because the steering angle of the vehicle is necessarily small in a high speed range, and large steering angles are required only when the vehicle is moving at a low speed, such a system in effect allows the vehicle operator to finely steer the vehicle in a high speed range, and highly sharply in a low speed range, thereby improving the handling of the vehicle in all speed ranges. Such variable gear ratio steering systems are proposed in commonly assigned U.S. Pat. Nos. 5,489,004 issued to Y. Shimizu et al. Feb. 6, 1996, and No. 5,503,239 issued to Y. Shimizu Apr. 2, 1996. The contents of these patents are incorporated herein by reference.
Such systems are found to be highly effective in improving the handling of the vehicle, but the Inventors of this application have discovered that the handling of the vehicle may be improved even further by modifying the gear ratio of the steering system under certain circumstances. For instance, the gear ratio should be increased when the road surface is significantly slippery.